The use of X-ray technology for providing two-dimensional images of breast tissue for diagnosis of carcinoma or other abnormalities is well known. X-ray imaging has a number of limitations which are universally recognized by radiologists. In particular, X-ray imaging of breast tissue has the inherent limitation that a mammogram provides only a two-dimensional image of a three-dimensional object. Thus, although a potential area of concern may be indicated on a mammogram, the elevation of the subject area within the breast may be uncertain, leading to a biopsy of broader scope than would otherwise be necessary.
In addition to conventional mammograms, apparatus has been developed that employs ultrasound technology for breast tissue imaging. Ultrasound imaging devices display echoes received from a piezoelectric transducer as brightness levels proportional to the backscattered echo amplitude. The brightness levels are displayed at the appropriate echo range and transducer position or orientation, resulting in cross-sectional images of the object in a plane perpendicular to the transducer emitting face.
Previously known ultrasound equipment, in the form of dedicated ultrasound breast imaging apparatus, have met with limited acceptance by the medical community. For example, Brenden U.S. Pat. No. 3,765,403 describes the use of ultrasound technology to provide direct and holographic imaging of breast tissue. That device requires the patient to lie prone on a patient supporting surface while her breast is immersed in a water-filled tank. Taenzer U.S. Pat. No. 4,434,799 describes an alternative device wherein the patient's breast is immobilized between an ultrasonic transducer and ultrasonic receiving transducer. Both of the systems described in those patents are dedicated ultrasound systems.
In addition to dedicated apparatus, hand-held ultrasound devices have found application in performing free-hand examinations. Free-hand examination using a hand-held ultrasound transducer is described, for example, Mendelson, "Ultrasound Secures Place In Breast Ca Management", Diagnostic Imaging, April 1991, pp. 120-129. A drawback of such freehand examinations, when used to supplement mammography, is the inability to provide geometric registration between the mammogram and ultrasound images. This lack of registration may result in the freehand ultrasound examination being directed at a different portion of the breast tissue than would otherwise have been indicated were geometric registration possible.
For example, recent studies have shown that over 10% of the masses detected with free-hand ultrasound and initially believed to be the mammographically detected mass, were subsequently found to represent different areas of the breast. Because ultrasound can depict 2-3 times more cysts than mammography, the possibility of characterizing a malignant lesion as benign is real.
In addition, the three dimensional shape of the lesions, as reported in Homer, "Imaging Features And Management Of Characteristically Benign And Probably Benign Lesions, Rad. Clin. N. Am., 25:939-951 (1987) and the increased vascularity associated with carcinoma, as reported in Cosgrove et al., "Color Doppler Signals From Breast Tumors", Radiology, 176:175-180 (1990), have been suggested to be added to the diagnostic criteria. Such volumetric spatial registration of the ultrasonic data with a mammogram cannot be accomplished with previously known ultrasound devices.
While there is recognition within the medical community of the advantages offered by ultrasound technology, the construction of conventional mammography and sonography equipment has prevented combination of these two technologies. In particular, polycarbonates such as Lexan.RTM., are typically used in mammography because of their tensile strength and transparency to X-ray. These materials are acoustically opaque.
On the other hand, the compression plates used in the conventional breast ultrasound devices, for example, Brenden U.S. Pat. No. 3,765,403, are composed of materials such as polystyrene or polyurethane, which have insufficient tensile strength for use in mammography equipment.
Because of their high densities, all of the materials potentially useful for the compression plates in mammography equipment have relatively high attenuation and reflection coefficients (table 1, below). These characteristics limit the use of ultrasound to low frequencies (3 MHz or below as described in Taenzer U.S. Pat. No. 4,434,799) and shallow depths. At 10 MHz and a 0.5 to 1 cm roundtrip path through a typical compression plate, the attenuation with most polymers would be 20-50 dB.
For any interface thicker than a quarter wavelength (several hundred microns, depending on the nominal frequency and acoustic velocity within the material) transmission loss must also be taken into account (which could exceed 50 dB). In addition, the impedance mismatch between the biological tissues, the compression plate and the transducer results in at least a 6 dB loss at each interface, or an additional total loss of 24 dB roundtrip. Since the total dynamic range is no greater than 100 dB for a typical ultrasound system, ultrasound imaging through previously known mammographic compression plates would be impossible.
In addition, since the acoustic propagation within the compression plate is substantially different than water or the coupling gel, refraction effects on each of the emitted waves from the elements of a phased array, would severely corrupt the beamforming process that assumes a constant velocity of 1540 m/sec.
TABLE 1 ______________________________________ Attenuation Coefficient Impedance Material (dB/MHz/cm) (Pa s/m) ______________________________________ Polyvinylchloride 11.1 3.4 Polybutane 6.1 3.2 Polyacetyl, Polyethylene, 2.5-3.3 2.2 Polypropylene Polyamid (Nylon) 1.1 2.9 Polystyrene 1 2.5 Water 0.02 1.5 ______________________________________
The lower frequencies used in the previously known ultrasonic devices would be inadequate for the diagnostic applications, which currently require 7-10 MHz transducers, yet this higher frequency requirement would increase the transmission loss by at least threefold (in dB). While it is possible to generate larger pulses in the transducer in the water bath approach, the low electro-mechanical efficiency results in heat generation. Placing the transducer directly upon the compression plate, and as a result in close proximity to the biological tissue, would require even higher energy pulses from each element. The resulting heat generation would cause damage and should be avoided.
Conway, "Occult Breast Masses: Use Of A Mammographic Localizing Grid For US Evaluation", Radiology, 181:143-146 (1991) and Brem and Gatewood, "Template Guided Breast Ultrasound", Radiology, 184:872-874 (1992), describe attempts to achieve spatial registration between a mammogram and an ultrasound image by cutting a hole in the compression plate of the mammography device to insert an ultrasound transducer. In Conway et al., a cut-open compression plate with a localization grid was used to allow acoustic transmission. Using the identical ultrasound device, the ultrasound study was performed in free-hand and through the localizing grid. Several additional X-ray exposures were needed to detect the lesion, replace the compression plate with the cut-out grid compression plate, then place the cut-out over the coordinates of the lesion. The grid positioned ultrasound detected 24% more lesions than free-hand. Ten percent were misidentified using free-hand ultrasound. None of the lesions were misidentified with the grid-guided compression.
The approach described in the foregoing articles has several practical drawbacks. For example, in Conway the patient's breast is marked with an indelible pen to assist the mammographer in repositioning the patient's breast on the localization grid after the compression plate is replaced by the cut-open compression plate used with the ultrasound transducer. As noted in that article, even the use of indelible markings on the patients skin does not absolutely guard against movement of the underlying breast tissue. In addition, the mammographer had to be present during the exam to ensure correct positioning, and the procedure length was significantly increased.
A cut-open compression plate with a localization grid suffers from the problem that the ultrasonic field is interrupted by the shadow of the compression plate, in all regions but the cut-out hole, thereby requiring prior knowledge of the interrogated lesion. As a result, in order to obtain a complete ultrasonic diagnostic image of the desired region of interest, it would be necessary to carry out a complex and burdensome manipulation of the mammographic compression procedure, and expose the patient to additional ionizing radiation.
In view of the drawbacks of previously known breast imaging apparatus and methods, it would be desirable to provide an apparatus and methods for providing geometrically registered X-ray and ultrasound images of breast tissue.
It would further be desirable to provide a compression plate that is both radiolucent and sonolucent, so that both a mammogram and ultrasound images of a patient's breast tissue may be obtained without moving the breast between the X-ray exposure and ultrasound imaging.
It also would be desirable to provide an apparatus for moving an ultrasound transducer through a predetermined path to generate a plurality of ultrasound images of breast tissue at preselected intervals.
It would be still further desirable to provide an apparatus capable of correlating geometrically registered X-ray and ultrasound images to provide holographic views of a patient's breast tissue.